Converters presently used to convert sulphur dioxide gas to sulphur trioxide gas are typically large cylindrical vessels comprising a shell containing a number of catalyst beds disposed one above the other. The processed gases pass through the catalyst beds in several, optionally desired sequences and are cooled between beds both to recover the heat generated in each bed and to assist in the kinetics and equilibrium of the reaction. Each bed is separated from other beds by division plates or membranes.
Classically, in sulfuric acid manufacturing plants, converters were fabricated from carbon steel, cast iron, and brick, when these materials were the only ones available. Carbon steel was used for the shell and cast iron posts, beams, and plates or sections were assembled inside the converter to support the many beds of catalyst.
In an alternative design, brick structures were erected for the same purpose, with steel being used for the external shell. At the time such converters were designed, manufacturing plants were small in capacity and gas strengths were low, which resulted in modest gas temperatures. In addition, platinum catalyst was used which tended to required operations at temperatures below those at which present-day catalysts operate. Vanadium catalyst, much larger plant capacities with higher internal pressures and much higher gas strengths have drastically increased the mechanical loads on such converter shells, while at the same time the conventional carbon steel becomes hotter and, hence, much weaker. Distortion of the vessel and, thus, leakage are, therefore, more common. However, such converters are well-known in the industry and a discussion of their features can be found in many references to sulfuric acid manufacture.
Sulfuric acid plant converters are essentially atmospheric pressure reactors which handle large volumes of gas, passing it vertically downward through shallow horizontal beds of catalyst with the gas removed between beds for cooling and then returned for the next conversion step. Typically, four beds have been used in most plants although some five bed converters exist. Converter diameters in modern plants may typically be 40 feet in diameter and 60 feet high while the beds contained in the converter will range from 2 to 4 feet thick. The remaining space is occupied by the plenums above and below the catalyst beds. The gas flows in a large plant will often require gas ducting in the 7 to 8 feet range. If this ducting is connected directly to the converter shell particularly above and below each catalyst bed, the converter height would have to be of the order of about 80 feet, which is not acceptable as being inconvenient as all the other plant equipment treating the gases are not more than 40 feet above grade.
The classic solution to this problem has been to make an elliptical or shallow rectangular opening in the converter above the catalyst bed and obtain the appropriate inlet area by using a broad opening. For example, a 4 feet duct could be replaced by a 2.5 by 6 feet rectangular opening. These transitions are a standard feature of the classic sulfuric acid plant and are referred to as "mouth organs". Aside from its cost, a mouth organ is complex, costly to maintain, and the transition to round ducts forces the other equipment around the converter to be located away from the converter shell. This further inflates the cost of the converter system and the plant. Where the plants are small, a simple round connection is feasible However, in the larger plants the mouth organ design is necessary but expensive.
The classic catalytic converter for sulphur dioxide conversion is also made of materials which have significant design restrictions on their use. Carbon steel which is used in the shells and transitions is very weak at the maximum temperatures obtained in modern reactors and deforms in an inelastic manner, using up the freedom for expansion in expansion joints and ducts and leads to cracking and leaks. It is also not strong enough for the internal catalyst bed supports. The catalyst is therefore held on high quality cast-iron grids which in turn are supported on cast iron posts. These posts are supported from foundation means under the floor and the structure is assembled starting from the bottom. The cast iron sections making up the bed are sealed using an appropriate asbestos rope or the like. Division between beds relies normally on steel plates supported by a network of posts with the division plates welded to minimize gas leakage.
In these designs, there is little possibility of re-arranging the cast iron sections or the divider plates to facilitate gas entry. The design is, thus, restricted in medium and larger size plants to the use of mouth organ transitions.
A newer approach to converter design is offered by McFarland in U.S. Pat. No. 4,335,076. McFarland switches from the classic materials for sulfuric acid plant converters to stainless steel throughout the converter and while using horizontal catalyst beds, supports them on flexible diaphragms which in turn are supported at the shell and an axial core tube by strength welding. The beds communicate with the gas supplies either from a peripheral gas distribution system, which relies on a nozzle located below the catalyst bed, or from the core of the converter with a radial gas outward flow into the space above the catalyst bed. This design concept therefore does not offer any help to the problem of entry to the catalyst bed through the side and, in fact, teaches that it should be avoided. It does, however, demonstrate an arrangement of the exit nozzle in which a round nozzle is located on the vessel wall below the catalyst bed.
When a multi-bed converter is required, there are not only the bed supports but also the dividers between the catalyst beds. For a 4 bed system there are therefore 7 membranes where all are horizontal and frequently supported. In the newer technology of McFarland the grids were replaced by stainless steel membranes which were much less rigidly supported but horizontal membranes were the basic standard.